Scale formation from the precipitation of inorganic chemical species in water has long been a problem. In residential experience, scale typically builds up as hard water deposits in pipes and other conduits, water softeners and the like, in areas where the water is "hard" that is, contains relatively high quantities of minerals. The carrying capacity of water pipes can be greatly reduced over time, and this phenomenon can cause substantial decrease in water pressure.
The problem of scale formation is particularly aggravated in industrial uses where large volumes of aqueous fluids containing dissolved minerals are handled. For example, scale commonly forms on the surfaces of storage vessels and conveying conduits for process water, and may break loose. These relatively large masses of scale deposits become entrained and ultimately damage and clog equipment such as tubes, valves, filters, screens and pumps. These crystalline scales may detract from the cosmetic appearance of a final product, such as paper products. In more severe cases, scale can clog heat exchange surfaces and thereby form a thermal insulating barrier which inhibits heat transfer efficiency, as well as impeding fluid flow in the system.
In the production and processing of petroleum, scale formation problems are particularly exacerbated by the high levels of dissolved inorganic minerals in the fluids encountered, and by conditions which favor the precipitation and growth of these minerals as scale. For example, injection waters used to pressurize formations often contain dissolved minerals which combine with other dissolved minerals in the formation brine to yield insoluble salts which appear as scale. The mixing of formation brine with water flood fluids can also lead to shifts in ionic strength and pH, which may also cause scale formation. Shifts in temperature and pressure in the near-wellbore region of the formation also cause scale to form.
Prevention and inhibition of scale formation is needed to avoid plugging of the producing formation and production equipment. One of the problems in treating scale is that it can be of many types. The compounds which form scale include, but are not limited to, calcium carbonate, magnesium carbonate, barium sulfate, calcium sulfate, other salts of alkaline earth metals, aluminum silicates, etc. Attempts to prevent or inhibit scale formation are complicated by the wide-ranging chemical and solubility characteristics of the various scale constituents. Often, more than one chemical treatment is used: for example, both acidic and alkaline soaks may be employed to address the variety of scales. Many techniques are known in the art which attempt to address scale prevention or inhibition.
One such method is outlined in U.S. Pat. No. 3,547,817 to Hansen, et al., assigned to Betz Laboratories. In the process of this patent, scale inhibition of an adsorbent is improved through the addition to the formulation of a metal ion. It is broadly stated that the invention applies to "any water soluble, dispersable adsorbents which are normally employed in the adsorption of inorganic contaminants." The adsorbents specifically mentioned and addressed, however, are phosphonic acid derivatives of the general formula: ##STR1## where X and Y are hydrogen or an alkyl group having between one to four carbon atoms, including the ammonium, amine and metal salts of these acids. Metal ions cited as having performance-enhancing effects on scale inhibition are Fe(III), Fe(II), Zn(II), Ni(II), Co(II), Cd(II), Cu(II) and Al(III).
The effectiveness of the U.S. Pat. No. 3,547,817 invention is attributed to the displacement of positive ions, such as Ba(II) for the case of BaSO.sub.4 scale, at the surface of the scale crystal by the aforementioned ions. This displacement supposedly changes the electrostatic potential of the surface in such a way so as to allow the adsorption of more inhibitor. Stated another way, the role of the added metal ion is to increase the amount of inhibitor adsorbed at the surface and thereby achieve better inhibition.
The use of nitrilotri(methylenephosphonic) acid (NTMPA) as a scale inhibitor is one example covered by U.S. Pat. No. 3,547,817. The present inventors have observed a small increase in the scale-inhibiting performance of NTMPA in the presence of, for example, Cu(II).
Other phosphonate scale inhibitors also show some extremely modest improvements in performance in the presence of some of the above mentioned metal ions, although Fe(III) and Al(III) have always been observed to have a deleterious effect on performance. Thus, it would be a great advance in the art of scale inhibitors if a material giving something more than a small incremental benefit could be found.
The use of ethylenediaminetetra(methylenephosphonic) acid (ENTMP) and other phosphonic acids alone as scale inhibitors is well known. For example, the following references described the uses of such compounds. (a) U.S. Pat. No. 3,867,286; (b) U.S. Pat. No. 3,792,084, both to Quinlan, which teach this material as a CaCO.sub.3 scale inhibitor and as a chelating or sequestering agent of metal ions; however complexes with metal ions are not taught as scale inhibitors; and (c) P. H. Ralston, "Inhibiting Water Formed Deposits with Threshold Compositions," Materials Performance, Vol. 11, No. 6(1972) pp. 39-44, which primarily discusses aminomethylenephosphonate (AMP); 1-hydroxyethylidene-1,1-diphosphonate (HEDP) and amine phosphate (AP).
Metal ion complexes of some of the phosphonic acids described above, including ENTMP, are also known, but not as scale inhibitors. Publications describing these complexes include, but are not limited to: (a) B. Spiess, et al., "Complexing Properties of Nitrilotri(methylenephosphonic) Acid With Various Transition and Heavy Metals in a 10:90 Ethanol-Water Medium," Polyhedron, Vol. 6, No. 6, (1987) pp. 1247-1249 (teaches the demetallization of wine); (b) M. T. M. Zaki, et al., "Metal Chelates of Phosphonate-Containing Ligands-IV: Stability of Some 1,6-Hexamethylenediamine-N,N,N',N'-Tetra(methylenephosphonic) Acid Metal Chelates," Talanta, Vol. 27, (1980) pp. 709-713; (c) European Pat. No. 186,990 to Christiansen teaching the use of alkyleneaminephosphonic acids together with polyalkylenepolycarboxylic acids as stabilization aids for peroxide systems in the presence of alkaline earth metal ions; (d) U.S. Pat. No. 3,833,486 relating to cyanide-free electroplating, to Nobel, et al.; and (e) European Pat. Document 256,284 A2 where a phosphonate sequestrant synergistically works with a soluble tin compound to extend the storage life of photoresist stripping solutions.